docs: counter: Document character device interface

This patch adds high-level documentation about the Counter subsystem
character device interface.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/7a71dff2868195901d074ae2ec1b5611838d4b24.1632884256.git.vilhelm.gray@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>